We propose to study (1) the effect of parathormone on the distal tubule reabsorption of phosphate in the rat and the possible role of carbonic anhydrase in the mechanism of PTH action, (2) the effect of calcitonin along the rat nephron, (3) the reabsorption of chlorine along the distal tubule; the effect of furosemide on glomerular filtration rate and its relation to the tubule glomerular feedback mechanism, (4) the inorganic transport mechanisms along the accessible proximal tubule, (5) the role of the pelvic urine in the concentration mechanism. The experiments will be done by recollection micropuncture on rats or isolated kidneys preparation, standing droplets in the proximal tubules and papillary perfusion. The analyses will make extensive use of electronprobe microanalysis of the Na, Cl, K, Mg, Ca, P and S content in picoliter volumes. We will continue to develop the electronprobe analysis of femtoliter volumes of samples kept frozen hydrated in the kidney and we will measure urea in picoliter samples with a new ultramicrofluorometric method.